1. Field of the Invention
The present invention relates to an apparatus for direct or indirect application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard.
2. Description of the Related Art
Apparatuses are used in so-called coating systems for providing a traveling material web, formed for example of paper, cardboard or a textile material, on one or both sides with one or several layers of the coating medium, for example color, starch, impregnating fluid or the like.
In the so-called direct application, the liquid or pasty coating medium is applied by an applicator system directly onto the surface of the traveling fiber material web, the latter being carried during application on a rotating countersurface, for example an endless belt or a backing roll. In the indirect application of the medium, the liquid or pasty coating medium is first applied onto a substrate, for example the surface of a backing roll configured as an applicator roll, and is then transferred from the substrate to the fiber material web in a nip through which the material web passes.
Known from U.S. Pat. No. 5,436,030 is an applicator for direct or indirect application of a liquid or pasty coating medium onto a traveling fiber material web, notably of paper or cardboard, which includes a metering system with a metering slot configured as an open-jet nozzle and formed between an approach-side lip and a departure-side lip. Owing to the configuration of the open jet of liquid or pasty medium proceeding through the ambient atmosphere, such applicators are also known by the trade name "Fountain Jet Flow Applicator." Of the two lips forming the metering slot, the one disposed on the side of the metering slot which, in the indirect application of the medium, the applicator roll rotates toward, or, in the direct application of the medium, the fiber material web runs toward, is called the approach-side lip. Accordingly, the second lip, located on the side of the metering slot from which the applicator roll or fiber material web departs from the applicator, is called the departure-side lip. In one embodiment of an applicator according to U.S. Pat. No. 5,436,030, a concavely curving guide surface for the liquid or pasty coating medium, bordering on the metering slot, is arranged on the free end of the approach-side lip or the departure-side lip.
An applicator comparable with the applicator according to U.S. Pat. No. 5,436,030 is also being marketed by the assignee of the present invention under the trade name "Jet Flow F." The German patent application P 44 32 177.5-51, assigned to the assignee of the present invention, discloses an adjustment system that is usable in a "Jet Flow F Applicator" and that allows adjusting the approach-side lip manually or automatically with respect to its clearance from the departure-side lip. Viewed across the width of the applicator, the clearance can be adjusted zonewise differently by use of actuators, so as to even out, e.g., localized production inaccuracies and/or realize a desired cross profile of the applied liquid or pasty medium. The cross profile may sectionally vary, for example, by being flattened at the edges of the fiber material web. Also possible is a coarse adjustment of the entire metering slot extending across the width of the applicator. Usually coordinated with each adjustment zone are one or several actuators that are placed at a specific sideways spacing relative to one another. The correct choice of the mutual clearances between the respective actuators is particularly important for optimum adjustment of a desired cross profile.
It has been found, though, that the adjustment of an individual actuator has, with respect to the longitudinal expanse of the metering slot or the machine width direction, an effect on the section of the flow of liquid or pasty coating medium. The flow of coating medium is normally to be influenced by adjacent actuators. Thus, the adjustment of an individual actuator affects the resulting cross profile. For example, the local enlargement of the metering slot depth occasioned by actuation of an individual actuator at a profile spot causes a local variation of the amount of coating medium traversing a preset unit area. The variation nonetheless can still be limited to the area of the adjustment zone of the operated actuator. Until the coating medium impinges on the surface of the traveling fiber material web or the applicator roll, however, sort of a spreading effect or evening effect occurs in machine cross direction in the jet of coating medium. Thus, a zonewise variation of the cross profile results from the originally locally restricted variation of the amount of coating medium. This effect on the cross profile, caused by an individual actuator, is hereinafter called "influential width."
Due to the mutual influencing of the respective actuators of a metering slot adjustment system and the resulting superimposition of the influential widths of adjacent actuators, an even cross profile can obviously be achieved only at relatively high control expense. Besides, the influential width resulting from an individual actuator depends upon various design-related technical parameters of the applicator and/or on the properties of the coating medium used, for example, total pump output or total volume flow of the applicator, operating speed, rheology of the coating medium, etc. The user of a finished applicator is no longer able to influence these parameters.